1. Field of the Invention
The present invention relates to the use of polymer dispersions as binder for sealing or coating compositions comprising at least one aqueous polymer dispersion, in which the polymerisate has functional groups, and at least one compound having two or more NH.sub.2 groups per molecule as binder for sealing or coating compositions for hydrophobic substrates. The present invention also relates to sealing or coating compositions comprising such a formulation, which are suitable for the coating of substrates, especially hydrophobic substrates such as polyurethane foams or EPDM components.
2. Description of the Background
Construction materials are often sealed with coating compositions in order to protect against environmental effects, including the processes of weathering. Examples of such environmental effects, especially outdoors, are sunlight, fluctuating temperatures, moisture, the chemical effects of increasing pollution, and mechanical effects as well. To be suitable, coating compositions must be stable toward the environmental conditions mentioned, i. e. they should have little water uptake tendency, high photostability, and high resistance to oxidizing conditions as may occur, in particular, when intense solar irradiation and polluted air act in combination. The compositions must also be of high mechanical stability, especially high tensile strength, so that the dimensional changes in the components, induced by fluctuating temperatures, do not lead to cracks being formed. Moreover, this is a property which they must not lose at low temperatures. Conversely, at high temperatures they must not become tacky, which would lead to unwanted soiling. A further quality desirable in the coating compositions is the ability to reflect light and heat, so as to avoid extreme temperature fluctuations in the construction materials. Good adhesion of the coating compositions to the substrate to be sealed is of particular importance, especially in the case of hydrophobic construction materials, such as polyurethane foams, or components made of EPDM plastics, to which the adherence of conventional coating compositions is poor.
The properties required of the coating compositions depend largely on the particular binder used. Thus W. A. Zissmann (Advances in Chemistry, Series 43, Gould edition, ACS Washington 1964) showed that good adhesion to substrates with low surface tension and low polarity (hydrophobic substrates) is achieved if the binder polymers are in turn of Low surface tension and low polarity. In general, the surface of polymer particles in polymer dispersions is populated by polar groups as a result, for example, of the adsorption of ionic surface active substances. This restricts the choice of possible polymers and of surface active substances. Moreover, the polymers used as binders should, as films, possess high tear strength and good extensibility. Further still, the binder polymers must possess a high pigment binding capacity in order to ensure the stability of the coating compositions.
There have been various reports of binder polymers which meet some of the properties desired. For instance, EP A 187 505 describes a binder polymer which couples heightened tear strength with comparatively high flexibility. This polymer is obtained by reacting a primary latex in the swollen state with a monomer having at least two ethylenically unsaturated bonds. However, this two stage process is highly complex and increases the cost involved in preparing such binderxpolymers.
U.S. Pat. No. 3,345,336 describes polymers having carbonyl groups which can be crosslinked with hydrazides of the polybasic carboxylic acids. In the crosslinked state these polymers are of high solvent resistance; they are used for treating leather, textiles and paper.
EP-B 258 988 describes a coating composition for polyurethane foams that includes a binder polymer having acetoacetyl or cyanoacetyl groups. A distinctive feature of the coating composition is its high degree of adhesion to the polyurethane foam. While the incorporated functional groups increase the tear strength of such binder polymers, there is at the same time a marked reduction in the extensibility of these coatings. This loss of extensibility is probably a consequence of crosslinking reactions by the reactive methylene and carbonyl groups of the abovementioned functional units present in the polymer.